In general, a shock absorber attached to a suspension system of an automobile or other vehicle has a cylinder having a fluid sealed therein and a piston connected with a piston rod, which is slidably fitted in the cylinder. The shock absorber further has a damping force generating mechanism comprising an orifice, a disk valve, etc. to generate a damping force by controlling a flow of fluid induced by sliding movement of the piston in the cylinder in relation to the stroke of the piston rod.
In a pilot-type hydraulic shock absorber disclosed in Patent Literature 1, for example, a back-pressure chamber (pilot chamber) is formed at the back of a main disk valve constituting a damping force generating mechanism. A fluid is introduced into the back-pressure chamber to apply the pressure in the back-pressure chamber to the main disk valve in the direction for closing the valve, and the pressure in the back-pressure chamber is adjusted with a solenoid valve (pilot valve), thereby controlling the valve-opening operation of the main disk valve. With this structure, it is possible to increase the degree of freedom for adjusting damping force characteristics.
In the hydraulic shock absorber disclosed in the Patent Literature 1, if it should become impossible to energize the solenoid valve, the valving element of the solenoid valve is moved to a fail-safe position by the spring force of a valve spring to abut against a fail-safe valve, thereby adjusting the flow path area mechanically with the fail-safe valve in place of the solenoid valve. Thus, an appropriate damping force can be obtained even in the event of a failure.